1. Field of the Invention
The present invention relates to an optical unit, as an optical unit, for projection apparatuses such as a liquid crystal projector apparatus, a reflection-type video display projector apparatus, and a projection-type rear projection television, which project a video image on a screen by using a light valve element, e.g., a liquid crystal panel or a video display element, to a projection-type video display apparatus, and to a color switching method. More particularly, the present invention relates to a technique for projecting a video image by using an electronic time-division optical characteristics switching element capable of periodically switching colors incident on a light valve element.
2. Description of the Related Art
A paper of “Throughput Color Switch for Sequential Color Projection (SID 2000 DIGEST) written by Gary D. Sharp et al. discloses an electronic time-division optical characteristics switching element (herein after, simply referred to as an optical characteristics switching element) capable of periodically colors incident on a light valve element by using polarizing rotation control elements for polarizing optical axis of specific wavelength bands such as an R polarizing rotation control element for rotating a polarizing axis. of R-light (red light) as P polarizing light and for converting the R-light into S polarizing light, a G polarizing rotation control element for rotating a polarizing axis of G-light (green light) as P polarizing light and for converting the G-light into S polarizing light, and a B polarization rotation control element for rotating a polarizing axis of B-light (blue light) as P polarizing light and for converting the B-light into S polarizing light.
As disclosed in the paper, light from a light source is reflected by a reflector and the reflected light passes through first a first lens array and a second lens array. The passing light is converted into the P polarizing light by a polarizing beam splitter (herein after, referred to as a PBS), is transmitted through a condenser, is incident on a transmission-type optical characteristics switching element. Thus, the R-light, the G-light, and B-light are sequentially outputted, are transmitted through the PBS. Those R-, G-, and B-lights are incident on a reflection-type liquid crystal panel and are reflected to the liquid crystal panel. When a video signal displays white, the P polarizing light is. converted into the S polarizing light by the liquid panel and is reflected by the PBS. The reflected light passes through a projecting lens, thereby obtaining an enlarged video image.
An optical unit in the disclosure is L-shaped as a whole. In the optical unit, a distance between the projecting lens and the liquid crystal panel is long and a product set is increased in size.
Further, in the liquid panel used for a projection-type video display apparatus, the response time of liquid crystal is long. Therefore, in the conventional projection-type video display apparatus for switching polarizing rotation control elements for R-, G-, and B-light which sequentially project the R-light, the G-light, and the B-light onto the overall screen of the liquid crystal panel, there is a problem in that the use efficiency of light is degraded.